1. Field of Invention
The invention generally relates to methods and apparatus for controlling an internal combustion engine of a motor vehicle, and more particularly to methods and apparatus for controlling an internal combustion engine in which an exhaust purifying catalyst having a function of storing oxygen is disposed in an exhaust system. In particular, the invention is concerned with methods and apparatus for controlling such an internal combustion engine that is temporarily stopped when predetermined conditions for stopping the engine are satisfied, and resumes its operation when the engine stoppage conditions are eliminated, so as to reduce NOx emissions resulting from the temporary stoppage of the engine.
2. Description of Related Art
In general, an exhaust purifying catalyst, such as a three-way catalyst, is provided in an exhaust system of an internal combustion engine of a motor vehicle, such as an automobile. The three-way catalyst of this type induces reactions between NOx and CO or HC as harmful components contained in exhaust gases of the engine, thereby converting these components into harmless N2, CO2 or H2O. The catalyst has a tendency of storing oxygen when an exhaust gas containing excessive oxygen, in view of a balance between an oxidizing component and a reducing component, or air passes through the catalyst. If the internal combustion engine is stopped for a long period of time, the exhaust purifying catalyst is naturally exposed to oxygen in the atmosphere, whereby the amount of oxygen stored in the catalyst reaches its saturation limit. As the amount of oxygen stored in the exhaust purifying catalyst increases, its capability of reducing or removing NOx may deteriorate. At the time of a start of the engine, therefore, the amount of fuel supplied to the engine is temporarily increased to be greater than a value corresponding to the stoichiometric air/fuel ratio, in accordance with the amount of oxygen stored in the catalyst, so that the catalyst is subjected to a reduction process utilizing the increased amount of fuel.
In view of increasing demands for saving of fuel resources and environmental protection (e.g., prevention of air pollution) in recent years, more and more attention has been drawn to so-called “eco-run” (economical-ecological running) vehicles and hybrid vehicles, in which an internal combustion engine is temporarily stopped when the vehicle is temporarily stopped at signals or in a traffic jam, or when the vehicle is preferably driven by an electric motor, rather than the engine. Since a temporary stoppage of the engine during an operation of the eco-run vehicle or hybrid vehicle only lasts for a short time period (e.g., not longer than about 10 min.), the exhaust purifying catalyst is kept from storing oxygen due to entry of air from the outlet of the exhaust system during such a short time period. However, the engine keeps moving/revolving for a while even after fuel supply to the engine is cut off or interrupted so as to stop the engine. During this idling of the engine, air containing no fuel component is introduced into the exhaust system, and oxygen in that air is stored in the exhaust purifying catalyst.
One example of a hybrid vehicle, which is currently manufactured and sold by the assignee of the present application, has a driving system as schematically illustrated in FIG. 1. With this driving system, the internal combustion engine may keep rotating even after fuel supply to the engine is interrupted so as to temporarily stop the operation of the engine. More specifically, the driving system of FIG. 1 includes an internal combustion engine 1, which is coupled to a generator 3 and an electric motor 4 via a drive coupling device 2 including a planetary gear set. A pair of drive wheels 6a, 6b are provided with respective axles 7a, 7b, which are connected to the axis of the motor 4 via a differential gear mechanism 8 and a transmission 5. The driving system of FIG. 1 is not provided with a clutch which was normally provided in a conventional vehicle driving system. With this arrangement, the speed of rotation and input torque (positive or negative) of the drive wheels 6a, 6b are controlled by combining the speeds of rotation and output torques of the engine and the electric motor and the speed of rotation and load (negative torque) of the generator by means of the planetary gear set of the drive coupling device 2. In some cases, such as when the vehicle is in a decelerating state or when the vehicle is being driven by the electric motor, no power needs to be generated by the engine, and fuel supply to the engine is cut off or interrupted. Even though the engine is allowed to stop rotating in these cases, the engine may still keep rotating while supplying no torque, depending upon the operating conditions of the vehicle. In this specification, “stoppage of the engine” is to be interpreted to include an idling state of the engine (i.e., a state in which the components of the engine (e.g., the pistons, etc.) are moving) with no fuel supplied thereto.
Referring again to FIG. 1, the driving system further includes a battery 9, or other storage device, an inverter 10, a catalytic converter 11, such as a three-way catalyst, provided in an exhaust system of the engine 1, and an electronic control unit 12. The electronic control unit 12 receives a signal indicative of an amount Dp of depression of an accelerator pedal, a signal indicative of a vehicle speed Sv, a signal indicative of a crank angle θe, a signal indicative of a temperature Te of the engine, and a signal indicative of a temperature Tc of the catalytic converter.